docs: add documentation for clientChunks option

Adds comprehensive JSDoc documentation for the clientChunks configuration option,
including usage examples, parameter details, and benefits of custom client chunking.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: hi-ogawa

packages/plugin-rsc/src/plugin.ts

@@ -180,7 +180,35 @@ export type RscPluginOptions = {
   }
 
   /**
-   * @experimental
+   * Custom chunking strategy for client reference modules.
+   *
+   * This function allows you to group multiple client components into
+   * custom chunks instead of having each module in its own chunk.
+   *
+   * @param id - The absolute path of the client module
+   * @returns The chunk name to group this module with, or undefined to use default behavior
+   *
+   * @example
+   * ```js
+   * clientChunks(id) {
+   *   // Group all client components in a specific route together
+   *   if (id.includes('/src/routes/dashboard/')) {
+   *     return 'dashboard-clients'
+   *   }
+   *   // Group third-party client components
+   *   if (id.includes('node_modules/@company/ui/')) {
+   *     return 'vendor-ui'
+   *   }
+   * }
+   * ```
+   *
+   * Benefits:
+   * - Reduces the number of HTTP requests by bundling related client components
+   * - Improves loading performance for route-based code splitting
+   * - Allows fine-grained control over client-side bundle optimization
+   *
+   * Note: Tree-shaking is still applied, so only the actually used exports
+   * from each module will be included in the final chunk.
    */
   clientChunks?: (id: string) => string | undefined
 }